Richard Thorne - US grants

This grant will continue an investigation of the interaction between waves and particles in magnetospheric plasmas with emphasis on the problem of oblique wave growth and the associated rate of particle scattering. Quantitative models will also be developed for the effect of such scattering on the average rate of loss of geomagnetically-trapped particles (the Van Allen radiation belts) the concomitant energy deposition into a planetary ionosphere, and the anticipated changes in the height-profile of ionospheric conductivity and its effect on the bulk transport properties of magnetospheric plasma. This work will be important to an understanding of the life cycle of the Van Allen radiation belts. It is thought that the plasma waves to be studied here are a major factor in controlling the Van Allen radiation belt populations.

This research seeks an understanding of the origin of magnetospheric plasma waves and their influence on the dynamical behavior of trapped particles. This will be accomplished through theoretical analysis and numerical computation. The path integrated growth rate and ultimate damping mechanisms for plasma waves will be studied in realistic model environments using the HOT RAY tracing code. This will establish the conditions for significant wave gain and the effectiveness of energy transfer, via the waves, to other components of the plasma. The stochastic acceleration of particles to high energy and precipitation loss to the atmosphere during resonant interaction with magnetospheric plasma waves will be studied by evaluating the rate of diffusion along the characteristic resonant diffusion surfaces. Several of the proposed topics are relevant to the ongoing GEM inner magnetosphere campaign, particularly as they address processes in the inner magnetosphere and geomagnetic storm conditions.

Three magnetic storms were identified by the joint SHINE-GEM-CEDAR magnetic storm campaign as candidates for a broad and inclusive study of the dynamics of magnetic storms. The periods to be studies are 10-11 May 1997, 24 Sept. - 1 Oct. 1998, and 18-31 Oct. 1998. This study will examine the spatial and temporal evolution of the electric fields in the inner magnetosphere during the three storm study periods. A global model for the storm-time electric field will be obtained by mapping the ionospheric electric field (as determined from ground based observations using the Assimilative Mapping of Ionospheric Electrodynamic - AMIE - technique) into the magnetosphere along magnetic field lines. The effects of penetration of electric fields to lower latitudes and hence the inner magnetosphere will be incorporated by a detailed analysis of field-aligned currents associated with the divergence of the ring current computed by the Ring Current-Atmosphere interaction Model (RAM). The RAM model will also be further extended to include the effect of wave-particle scattering of energetic O+ ions. The model results will be compared with satellite data.

The principal objective of this project is a detailed understanding of the origin of magnetospheric plasma waves and the influences these waves have on the dynamical behavior of trapped magnetospheric particles. The path integrated growth rate and damping mechanisms for plasma waves will be studied in realistic model environments using a hot ray tracing code. This will establish the conditions for significant wave gain and the effectiveness of energy transfer, via the waves, to other components of the plasma. New codes will be developed to evaluate the rates of pitch-angle scattering and energy diffusion of resonant radiation belt particles with each important class of wave. Quantitative calculations will be made of the rate of particle precipitation loss to the atmosphere and of the time-scale for stochastic acceleration under different levels of geomagnetic activity. The time-scales for loss and local acceleration will be incorporated into a radial diffusion code, to model the structure of the electron radiation belts and their variability during geomagnetic activity. The excitation of plasma waves and their effect on energetic ion and electron losses will be examined as will the mechanisms responsible for acceleration of energetic electrons to relativistic energies during magnetic storms. The project will lead to a global understanding of dynamics of the ring current during magnetic storms. A synoptic model of the wave-particle processes will be developed for future integration into a Geospace General Circulation Model.

Chorus is a highly non-linear whistler mode electromagnetic emission, which is excited in the low-density region exterior to the plasmapause under geo-magnetically active conditions. This important magnetospheric wave can interact with outer radiation belt electrons, causing pitch-angle scattering loss and energy diffusion. Previous calculations of the effect of wave-particle scattering has generally been performed using a quasi-linear approach. This project will quantify the effects of scattering by non-linear discrete chorus elements and compare the results with quasi-linear theory. The approach will be to carry out test particle calculations for a large number of electrons in a simulated wave field, which will be constructed including the effect of Landau damping of chorus during propagation to higher latitude. The results will subsequently be used to construct the global distribution of precipitation flux due to chorus scattering and the associated changes in ionospheric conductivity.

The research is relevant to the Geospace Environment Modeling (GEM) Magnetosphere-Ionosphere coupling campaign and to the future Radiation Belt Storm Probes (RBSP) mission of National Aeronautics and Space Administration (NASA). It is also central to the Challenge "Understanding the basic physical principles manifest in processes observed in solar and space plasmas" listed in the 2002 NRC (National Research Council) Decadal Report.

This project will develop a Radiation Belt Module (RBM) for the Geospace environment Modeling (GEM) Geospace General Circulation Model (GGCM). The RBM will facilitate a major advance in our understanding of energetic electron non-adiabatic dynamics, since, for the first time, all dominant physical process, which affect radiation belt electrons, will be simultaneously evaluated. The RBM will be driven by the coupled Rice Convection Model (RCM) of the inner magnetosphere and magnetohydrodynamic (MHD) codes to describe the global magnetosphere. It will consequently provide the capability of predicting changes in the radiation belts based solely on solar disturbances.

In the collisionless magnetospheric environment, wave particle interactions provide the dominant mechanism for pitch-angle scattering, energy diffusion and anomalous cross-field transport. The current understanding of such processes has now matured to a level where realistic quantitative models can be constructed. In the RBM, such processes will be quantified by 3-D diffusion coefficients, based on the anticipated power spectral density of scattering waves. Radial diffusion rates will be obtained from the properties of ULF waves simulated by the MHD code. A parameterization scheme for the global distribution of VLF waves and EMIC waves will be developed, based on the flux of injected ring current electrons and ions provided by the RCM. A quasi-linear diffusion code will be used to evaluate the rates of pitch-angle scattering and energy diffusion on a global scale. Numerical integration of the 3-D diffusion equation will provide a simulation of the response of radiation belt electrons to solar disturbances. The proposed study is central to three challenges in the 2002 NRC Decadal report: (1) Understanding the space environment of Earth and other solar system bodies and their dynamical response to external and internal influences, (2) Understanding the basic physical principals manifest in processes observed in solar and space plasmas, and (3) Developing near real-time predictive capability for understanding and quantifying the impact on human activities of dynamical processes at the Sun, in the interplanetary medium, and in the Earth's magnetosphere.

The UCLA Visualization Portal facilities will be used to construct 3-D movies of the variability of radiation belt electrons during storms. Results from the study will be integrated into courses for a new Collaborative Laboratory and Space Plasma Physics Interdisciplinary Degree Program at UCLA, and will be shown at UCLA visualization portal for graduate students.

Diffuse auroral precipitation provides a major source of energy input to the Earth's upper atmosphere. The precipitated particles lead to changes in the rate of ionization, which can modulate large natural current systems and induce electrical currents along transmission lines and pipelines. The additional ionization can also result in chemical changes to the neutral atmosphere, which has been linked with ozone-depletion. The precipitated particles also lead to changes in the electrical conductivity of the ionosphere, which maps along magnetic-field lines and further affect the transport of plasma in the magnetosphere.

The diffuse aurora is primarily caused by rapid pitch-angle scattering of plasma sheet electrons. Because the magnetosphere is essentially collisionless, the only viable mechanism that can cause pitch-angle scattering is wave-particle interactions. Previous studies have shown that scattering by both electrostatic electron cyclotron harmonic (ECH) waves and whistler-mode chorus could be important, but there is still no general consensus on the dominant process. This project will examine the roles of ECH and chorus waves by looking at the rate of wave-induced particle scattering by both classes of wave. Pitch-angle diffusion rates will be calculated with existing codes, based on a comprehensive analysis of the spectral properties of chorus and ECH waves observed by the CRRES satellite in the outer magnetosphere, under different levels of magnetic activity. Computed rates of electron scattering will be used to determine the equilibrium pitch-angle distribution and the rate of precipitation loss to the atmosphere compared to the limit imposed by strong diffusion. The scattering results will be used, together with statistical data on the global distribution of trapped electrons in the near-Earth plasma sheet, to model the global distribution of diffuse auroral emissivity. This study will provide a quantitative understanding of how microphysical processes regulate the transfer of energy in geospace. The proposed research is central to the primary science objectives of the new GEM Focus Group on Diffuse Auroral Precipitation.

Much of the research will be undertaken by a graduate student and a young research scientist. The scientific results obtained will be made available to other members of the magnetospheric community interested in modeling the global distribution of the ring current electron population and diffuse auroral precipitation under different geomagnetic conditions. The results of the study can be used to model the global distribution of ionospheric conductivity, which regulates the rate of magnetospheric convection, and is critical for the development of a Geospace General Circulation Model. Quantification of electron scattering rates in the near-Earth plasmasheet is also important for understanding the evolution of electron flux and pitch angle distributions during injection events, which provide the source of free energy for the excitation of waves affecting energetic radiation belt dynamics. The important physical processes studied at Earth, can be applied to other magnetized planets such as Jupiter and Saturn, where similar scattering can occur.

Waves exist in space plasmas just as in the oceans and the atmosphere. In these plasmas, collisions between charged particles are rare. As a result, plasma waves are a major means of transferring energy from one charged particle population to another. Charged particles "surf" the waves. To first order, those that are moving slightly faster than the waves are energized, while those moving slower lose energy to the waves causing them to grow. There are a wide variety of plasma waves with different properties and different source mechanisms. Three of these (plasmaspheric hiss, chorus, and electromagnetic ion cyclotron (EMIC) waves) are widely believed to play significant roles in the depletion of the electron radiation belts but how this happens and how each contributes with local time and radial distance are still-open and strongly debated questions of fundamental importance. During their interactions with the waves, electrons are scattered out of their trapped orbits and sent on trajectories into the dense atmosphere where they are lost through collisions. The work will independently examine experimental observations and, most importantly, use theoretical tools to understand the interactions leading to the precipitation. The science questions to be addressed in this proposal are particularly important, since electron precipitation leads to chemical changes in the upper atmosphere, and is critical in regulating ring current and radiation belt electron dynamics. The grant will support the further training and development of a promising female early-career scientist. The results will be useful to the broader space physics and upper atmosphere communities, to researchers studying the chemistry of the middle atmosphere, and for space environment applications, such as active mitigation techniques for both natural and artificial radiation in space.

Testing theoretical ideas about particular wave-particle interactions and the variations in the space environment that effect them has been difficult because the waves are measured at large radial distances in the magnetosphere while the electron precipitation that they produce must be viewed from low-earth orbit. To complicate matters, the mix of plasma waves depends on the radial distance and magnetic local time but in addition is an as yet to be determined function of the severity of space weather storming, and the phase of the storm. The principal investigator (PI) has developed an innovative technique to analyze the physical relationship between wave intensity and wave-driven electron pitch angle scattering loss, which can be directly implemented using conjugate observations from near-equatorial and low-altitude satellites. This project, which uses both theory and observation, will provide a definitive understanding of the quantitative contribution of each type of plasma wave to electron precipitation within various energy ranges and in different L-MLT regions. The results will provide a highly important contribution to our wider understanding of the mechanisms that regulate the hazardous radiation environment surrounding the Earth.